1. Field of the Invention
The present invention relates to a heater suitable to be used as a ceramic heater used in a fixing apparatus mounted to an image forming apparatus such as an electrophotographic copying machine and an electrophotographic printer, and to an image heating apparatus having the heater mounted thereon, such as a fixing apparatus.
2. Description of the Related Art
Image forming apparatus employing an electrophotographic system have been developed for higher speed, higher function, and colorization, and various types of copying machines and printers have been placed on the market.
On the copying machines and printers employing the electrophotographic system, there is mounted a fixing apparatus for heating an unfixed toner image formed on a recording material to fix the toner image onto the recording material. As one heating system for the fixing apparatus, there is a film heating system.
The film heating system is a system in which a ceramic heater is provided on an inside surface of the cylindrical shape of a fixing film and a pressure roller is provided at a position opposed to the ceramic heater across the cylindrical film to bring the fixing film into contact with the recording material by pressing the pressure roller toward the ceramic heater so that heat of a ceramic heater is applied into the recording material. The cylindrical film (fixing film) is made of a heat resistant resin or metal based material.
The ceramic heater used in the fixing apparatus employing the film heating system often includes, on a heater substrate made of ceramics, a heat generating resistor formed of an electrical resistor, a power feeding electrode made of silver and the like, and an insulating layer made of glass for protection of the heat generating resistor. Further, in most cases, power is fed to the ceramic heater by a method of bringing a connector including a power feeding contact into press contact with the electrode on the heater substrate, thereby forming an electrically conductive path.
In the ceramic heater, in most cases, the heat generating resistor and the electrode are formed on the same surface of the heater substrate. However, in some cases, in order to reduce cost by using general connectors or reducing the substrate width, the heat generating resistor and the electrode are formed on opposite surfaces of the heater substrate, respectively. In the ceramic heater with such a configuration, a through hole is formed in the heater substrate so that a conductive path is formed between the heat generating resistor and the electrode.
Japanese Patent Application Laid-Open No. 2002-299014 discloses a ceramic heater in which heat generating resistors having different heat generation areas are formed on both surfaces of the heater substrate, and a through hole is used to feed power from one of the surfaces. It is known that, when small-sized recording materials are successively printed by a printer mounting a fixing apparatus employing the film heating system at the same printing interval as that for large-sized recording materials, a temperature of an area of the ceramic heater in which the recording material does not pass (non-sheet passing area) excessively rises (non-paper passing portion temperature rise).
In the configuration of the ceramic heater disclosed in Japanese Patent Application Laid-Open No. 2002-299014, in order to address the problem called the non-paper passing portion temperature rise, heat generating resistors having different lengths are provided on both surfaces of the heater substrate, and the heat generating resistors are selectively used depending on the paper size. Further, when two heat generating resistors are formed on the same surface of the heater substrate, it is necessary to increase the width of the heater substrate by the width of the respective heat generating resistors and a distance for ensuring insulation between the two heat generating resistors. However, when the heat generating resistors are divided onto front and back surfaces of the substrate, increase of the substrate width can be prevented.
By the way, in the ceramic heater in which power is fed to the heat generating resistors on both surfaces of the heater substrate via the through hole as described above, as compared to a general integrated circuit device, it is required to cause a larger amount of current to flow. In some cases, the through hole abnormally generates heat to be burned, which may cause conduction failure.
Japanese Patent Application Laid-Open No. H04-185455 discloses a configuration in which, when power is fed to the heat generating resistors on both surfaces of the heater substrate via the through hole, multiple through holes are used to prevent conduction failure caused by the burning of the through hole.
As described above, in the ceramic heater in which power is fed to the heat generating resistors via the through hole, it is demanded to prevent burning of the through hole and conduction failure caused by the burning. Even in the case of feeding power via multiple through holes, a further improvement is demanded.